It is conventional to employ straight dipole antennas for the transmission and/or reception of signals in FM bandwidth in the range from 88 to 108 MHz. Half-wave dipoles, i.e., wherein the tip-to-tip electrical length of the dipoles is one-half wave length at the operating frequency, are typically employed for FM reception. For example, a straight dipole at the low end of the FM range and which is resonant at 88 MHz, has a tip-to-tip electrical length, for a half-wave dipole, which is approximately 67 inches. At mid-range, i.e., 98 MHz, tip-to-tip length of the half-wave straight dipole is 59 inches while the tip-to-tip length of a half-wave straight dipole resonant at 108 MHz is approximately 54.5 inches. It is quite common to use three active dipoles coupled to form an antenna array to obtain signal gain sufficient for the desired FM reception. Such dipoles require a large amount of mounting space and typically cannot be accommodated within confined areas, such as apartments, hotel rooms, trailers and other mobile units, or in any application where space is at a premium. In addition, straight dipoles of the type described hereinabove are highly directional and require the exercise of some care in order to set the antennas up properly for best reception or, alternatively, require expensive antenna rotating units, especially in cases where the FM transmitters within a particular area are clustered in a circular mounter about the FM receiving unit.
It is thus extremely desirable to be able to provide an antenna design of significantly reduced size and weight and having a desirable omnidirectional pattern so as to avoid the careful set up of an antenna or for that matter the continual azimuthal alignment of an antenna each time reception from a different transmitter is desired.
The present invention is characterized by providing a highly miniaturized light-weight, rugged FM antenna employing a novel curved overlapping dipole design in which the space occupied by the antenna is significantly reduced, as compared with straight dipoles. In addition to the above, a solid state amplifier is employed which is so designed as to provide adequate gain over the entire FM band in spite of the fact that only one dipole structure is employed in the antenna design. The physical positioning of the solid state amplifier in extremely close proximity to the antenna feed points to permit direct connection therebetween, eliminates transmission line losses which would otherwise occur.
The solid state amplifier is sufficiently miniaturized so as to occupy an insignificant amount of space within the dust proof, weather-proof housing provided to completely enclose the antenna. The remotely located power supply for the solid state amplifier is coupled to the solid state amplifier through a coaxial cable which is utilized in a novel diplexed manner, coupling DC power through the amplifier and simultaneously coupling the amplified output signals back to the power supply whereupon the received signal is converted from 75-ohm unbalanced to a 300-ohm balanced signal which is highly suitable for connection to the antenna input terminals of an FM tuner/receiver.
The stagger-tuned design of the solid state amplifier serves to provide adequate gain over the entire FM band to further assure that the desired omnidirectional E-plane pattern is obtained, thereby substantially totally eliminating any need for rotating or otherwise changing the position of the antenna assembly for purposes of improving signal reception, even in cases where transmitting stations are located in different angular positions about an azimuthal plane.
The light-weight housing is formed of a rugged and yet aesthetically pleasing plastic material in substantially disked shaped form and is adapted to be mounted quickly and readily by a supporting bracket on either interior or exterior supports and is further adapted to receive short leg elements which may be releasably secured thereto for sturdily supporting the structure for any flat or reasonably flat surface.
In one preferred embodiment, the curved dipole arms which are arranged in substantially coplanar fashion and are tuned preferably to the mid point of the FM band (of the order of 98 MHz) and may be securely and compactly housed within the protective housing whose external diameter is only 11 inches and whose thickness or height is of the order of 1 inch. The dipole antenna structure is preferably comprised of twin lead secured along the interior cylindrical wall of said housing.
The light-weight small attractive appearance makes it possible to place the antenna in regions where space is limited, such as apartment, hotel rooms, homes and the like. The attractive appearance does not detract from the use and placement of the antenna as part of the decor. In buildings employing steel superstructures the antenna is small enough to be conveniently placed next to a window or even outside a window to place the antenna in a position where the steel superstructure does not affect signal reception.
The antenna is provided with a versatile mounting bracket to facilitate its mounting on walls in attics and on masts indoors or outdoors. The light-weight even permits its mounting by means of a double sided pressure sensitive adhesive strip.